Rain in boiler vent

[rockycmt]

I have a NG boiler in my below grade basement. Live in NY area. Have had 2 major rain storms this past month. Both caused some water entry near my boiler. I seen water dripping from a horizontal section of my vent. I am not sure if it was rain entering from the chimney opening or groundwater accessing the bottom of the system. There is a vent cover on top of chimney but the rain was sideways for days. Is this a common issue? What is the usual cause. I am leaning toward ground water against the house.

[Dana]

If it's a masonry lined exposed brick chimney, masonry is porous, and some seepage into the flue and runng down the vent piping during saturating rains events is common enough. If you have a steel chimney liner this shouldn't be an issue if you have a decent venturi-type chimney cap as opposed to an umbrella-top & screen type. If there's no overhanging eaves or the surface grade isn't sloped away from the house, ground moisture could conceivably be the culprit, but it's a matter of how much area of the masonry is exposed to the moisture.